In posterolateral fusion procedures, bone graft is placed on the posterior side of the spine between the transverse processes of vertebrae. The graft promotes bone growth between the vertebrae. When the new bone is fully formed it fuses two or more vertebrae together, increasing the stability of the spine. However, it can be difficult to prepare the vertebrae so that the bone graft properly promotes fusion.
Spinal rods are used to stabilize the spine while the bone graft heals. The procedure for implanting a spinal rod often involves inserting pedicle screws that will secure the spinal rod to two or more vertebrae. To install a pedicle screw, a targeting needle that includes a stylet and cannula is advanced to a desired pedicle and tapped into the bone using a mallet. When the targeting needle is at a desired depth, the stylet is removed leaving the cannula. A k-wire is guided down the cannula and inserted into the bone. Once the guide wire is in place, the cannula is removed. A series of dilators are used to distract tissue about the k-wire. A bone awl is passed over the k-wire and used to puncture through the cortical layer of the vertebra to create a hole not exceeding the depth of the k-wire. A tap is then guided over the k-wire to tap the hole formed by the bone awl. Once the tap is removed, the bone screw can then be lead down the k-wire to the hole and screwed into place.
The tip of a k-wire is very sharp and can easily pass through bone. If the awl, tap or bone screw hit a kink in the k-wire, they can cause the k-wire to push through the vertebra further than intended. Consequently, surgeons must work slowly when guiding tools to a surgical site.